Now all items are coming down as DRM books, where that was not the case on my former PC. I am really puzzled by all this. Any clues?

All items most likely came down with drm before, this is how Amazon does the vast majority of their books. You must have previously installed a 3rd party plugin to remove drm as books are added. We can't provide detailed instructions about drm removal in this forum. Go to where you grabbed the plugin and ask for help there. If you don't remember where you got the plugin google apprentice alf's blog.